Desired hardness and strength properties of metals, particularly ferrous metals and especially metal alloys such as carbon steel and alloy steel, are secured by heat treatment of the metal object. The properties usually depend upon establishment of certain physical structures in the metal. The production of the desired physical structures is obtained by heating the metal to a temperature where the structure is present, then by arresting at the desired point the changes in the internal structure which take place during cooling of the metal from high temperatures. Quick cooling by quenching the heated object in a quenching medium makes it possible to arrest the physical changes at the desired point during cooling.
Quenching in the quenching medium is carried out in such a manner that the physical changes in the metal are arrested at the desired point, usually at the point at which maximum hardness is obtained. Subsequently, the heat treated and quenched object may be subjected to treatment at lower temperature (annealing or tempering) to provide the desired degree of toughness and ductility.
For many years mineral oil based quenching fluids have been used. Previously, aqueous quenching media were employed. The aqueous fluids provided extremely rapid cooling setting up excessive amounts of internal stress in the object. Mineral oil based fluids avoided this difficulty. Quenching fluids should in use be stable for a prolonged period at relatively high temperatures. A common problem of oil based fluids is that during use sludge will form which tends to stick to the metal object.
Publications which deal with the problem of sludge formation are for example WO-A-03052146 and U.S. Pat. No. 6,239,082. WO-A-03052146 describes a quenching fluid consisting of an oil and an alkali metal salt derivative. The examples of WO-A-03052146 disclose quenching fluids containing at least a mineral oil, para-dodecyl phenol and polyisobutylene. U.S. Pat. No. 6,239,082 describes a quench oil consisting of a mineral oil, polyisobutylene and polyisobutylene succinic anhydride. These publications show that the mineral oil needs to be combined with para-dodecyl phenol in order to improve oxidation stability and avoid sludge formation. Poly iso butylenes are added to improve the cooling capacity of the fluid.